On 10-Dec-07 23:41:34, Neil Stewart wrote: > I would like to make a new vector with values taken from two > existing vectors conditional upon a third. At present I am > doing this with a for loop but wonder if there is not a neater > way, given some of the very neat things R can do. > > a<-rep(c("A","B"),50) > b<-rep(1,100) > c<-rep(2,100) > > a is thus "A" "B" "A" "B" "A" "B"... > b is thus 1 1 1 1 1 1 ... > c is thus 2 2 2 2 2 2 ... > > I want d[i] to be b[i] if a[i]=="A" and c[i] if a[i]=="B". > I'm current using a for loop: > > d<-rep(0,100) # initialise d > for(i in 1:length(a)) {if(a[i]=="A") d[i]<-b[i] else d[i]<-c[i]} > > d is thus 1 2 1 2 1 2 1 2 1 ... > > Is it possible to do something simpler, say along the lines > of the c-style ?: conditional statement, or at least avoiding > the for loop. > > d <- a=="A"?b:c # doesn't work, but you get the idea > > Thanks in advance, > Neil
You could use one of at least two simple approaches. When b and c are numeric, then you could use d <- b*(a=="A") + c*(a=="B") since the numeric operation coerces the logical 'a=="A"' into 1 or 0 according as it is TRUE or FALSE, and vice versa. If b and c are not numeric (and indeed generally, but the above is neater for numeric variables), then: d <- b ; d[a=="B"] <- c[a=="B"] which simply over-writes the b-values in d where a=="B". Hoping this helps, Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <[EMAIL PROTECTED]> Fax-to-email: +44 (0)870 094 0861 Date: 10-Dec-07 Time: 23:55:45 ------------------------------ XFMail ------------------------------ ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.